Acoustic well logging is a well-developed art. Details of some existing acoustic logging tools and techniques are set forth in A. Kurkjian, et al., “Slowness Estimation from Sonic Logging Waveforms”, Geoexploration, Vol. 277, pp. 215-256 (1991); C. F. Morris et al, “A New Sonic Array Tool for Full Waveform Logging,” SPE-13285, Society of Petroleum Engineers (1984); A. R. Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . ” SPE 20557, pp. 267-282 (September 1990); and C. V. Kimball and T. L. Marzetta. “Semblance Processing of Borehole Acoustic Array Data”, Geophysics, Vol. 49, pp. 274-281 (March 1984). An example of an acoustic logging tool is provided in U.S. Pat. No. 6,661,737.
To summarize, acoustic logging tools typically include an acoustic source (transmitter), and a set of receivers that are spaced several inches or feet apart. An acoustic signal is transmitted by the acoustic source and received at the receivers of the borehole tool which are spaced apart from the acoustic source. Measurements are repeated every few inches as the tool descends or ascends in the borehole. The acoustic signal from the source travels through the formation adjacent the borehole to the receiver array, and the arrival times and perhaps other characteristics of the receiver responses are recorded.
Several different types of dipole acoustic transmitters have been used in acoustic logging tools. Some dipole acoustic transmitters are actuated using piezoelectric forces, while others are actuated using electromagnetic forces. Piezoelectric-based acoustic transmitters suffer high electrical impedance at low frequencies and are unable to deliver large pulse pressures at low frequency. Further, the displacement needed for low frequencies can damage the piezoelectric materials. While electromagnetic-based acoustic transmitters can overcome high electrical impedance physics at low frequency, it is not a trivial task to design a dipole acoustic transmitter that can deliver efficient, high-purity dipole performance over a range of frequencies.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.